New US Environmental Protection Agency (EPA) regulations that go into effect in 2007 and 2010 will require that the performance of diesel engine after-treatment systems be monitored by on-board diagnostics (OBD) systems, and that the OBD systems be capable of detecting malfunctions in or failures of the after-treatment systems. Direct on-board measurements of a diesel engine's NOx reduction efficiency are presently not technically or commercially viable, because existing NOx sensors have inadequate sensitivity and durability, and because they are expensive. Thus, at present, no robust OBD strategy for diesel engine NOx adsorber catalysts is available to meet the new EPA regulations.